Protein Glycosylation: Implications for In Vivo Functions and Therapeutic Applications - Advances in Biochemical Engineering Biotechnology - page 169

of human CD2 interacts with the polypeptide part of the molecule. As a result
the mobility of the proximal glycan residues is restricted. The glycan counter-
balances an unfavourable cluster of positive charges from surface Lys, through
hydrogen bonds and van der Waals contacts, and thus plays a role in stabilizing
the native receptor structure [94]. The solution structure of CD58 revealed that
the N -glycan, located opposite to the binding site, is not directly involved in
ligand binding, and a single Gn residue stabilizes the receptor structure [95].
Hydrogen-deuterium exchange kinetics reveals that, while glycans had little
overall effect on the three dimensional structure of the glycoprotein,there was a
global decrease in dynamic fluctuations due to hydrophobic and hydrogen bond
interactions which correlated with an increase in stability by ~1 Kcal mol -1 [63,
96].The same glycan on different proteins may have quite different effects depen-
ding on the orientation with respect to the polypeptide [97]. Also, different gly-
coforms of a protein may display quite different orientations of the glycan with
respect to the protein,thus conferring different conformations [94].For example,
the conserved complex glycan on each heavy chain of IgG Fc C H 2 domain occu-
pies the interstitial space between the domains and stabilizes Fc hinge confor-
mation.The antenna of each glycan,in particular the terminal Gal,interacts with
hydrophobic and polar residues on the domain surface. Loss of the two terminal
Gal, as in patients with rheumatoid arthritis, results in a loss of interaction with
the C H 2 domain surface.This displaces and exposes the N -glycan,giving them the
potential to be recognized by endogenous Man-binding proteins [98].

3.3
Receptor Functioning

The exact role of N -glycans in the function of glycoprotein receptors is varied.
Receptor glycosylation is generally thought to mediate correct folding and
insertion of the protein into the extracellular membrane, as in the case of FSH
receptor [99]. Glycosylation is also thought to be important in ligand binding in
some receptors, such as somatostatin receptor [100], vasoactive intestinal pep-
tide (VIP) receptor [101], and in the functioning of rhodopsin, the dim-light
photoreceptor of the rod cells [102], CD2 receptor [103], and in the -subunit of

the human granulocyte macrophage colony stimulating factor (GM-CSF) re-
ceptor [104].
In human Ca 2+ receptor [105] and in FSH receptor [99] N -glycosylation is
essential for cell surface expression. Similarly, for the mature LH/CG receptor to
reach the cell surface,post-translational processing of high Man glycans to com-
plex glycans is essential [106].Inhibition of N -glycosylation prevents cell surface
presentation of VIP receptor as in FSH receptor [101]. Changes in cell surface
expression likely reflect abnormal folding of the nonglycosylated receptor pro-
tein or decreased stability or transport. In some cases, though the receptor
contains more than one glycosylation site, glycans at one of the sites alone is
adequate for the proposed role as in FSH receptor [99],human transferrin recep-
tor [107] and rhodopsin [102].

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